Lines 1198-1247: why, in line 1227, is Elizabeth "in agony"?

This question is for The Crucible Act 3

Because of what John said I should forgive you, if you'll not forgive yourself

Answer: Elizabeth is in agony because she doesn't want to admit she already knows about Proctor's relationship with Abigail.

Explanation:

Elizabeth is being questioned by Danforth about whether or not she suspected that Proctor (her husband) had been cheating on her with Abigail. She doesn't want to say that to prevent him being punished for it. Little does she know that Proctor already confessed, and her denying the truth puts him at risk.